pgsql: Cygwin and Mingw floating-point fixes.

Cygwin and Mingw floating-point fixes.

Deal with silent-underflow errors in float4 for cygwin and mingw by
using our strtof() wrapper; deal with misrounding errors by adding
them to the resultmap. Some slight reorganization of declarations was
done to avoid duplicating material between cygwin.h and win32_port.h.

While here, remove from the resultmap all references to
float8-small-is-zero; inspection of cygwin output suggests it's no
longer required there, and the freebsd/netbsd/openbsd entries should
no longer be necessary (these date back to c. 2000). This commit
doesn't remove the file itself nor the documentation references for
it; that will happen in a subsequent commit if all goes well.

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/72880ac182c8f3769f0be868f77166994282cb2a

Modified Files
--------------
configure                     | 19 +++++++++++++++++++
configure.in                  | 13 +++++++++++++
src/include/port.h            |  5 +++++
src/include/port/cygwin.h     |  8 ++++++++
src/include/port/win32_port.h | 17 ++++++++++++-----
src/port/strtof.c             | 10 +++++++++-
src/test/regress/resultmap    |  7 ++-----
7 files changed, 68 insertions(+), 11 deletions(-)